Photon-scattering experiments at γelbe and at HIγS Data analysis Results Comparison of experimental results with model predictions

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1 Text optional: Institutsname Prof. Dr. Hans Mustermann Mitglied der Leibniz-Gemeinschaft Pygmy dipole strength in 86 Kr and systematics of N = 5 isotones R. Schwengner 1, R. Massarczyk 1,2, G. Rusev 3, N. Tsoneva 4, D. Bemmerer 1, R. Beyer 1, R. Hannaske 1,2, A. R. Junghans 1, J. H. Kelley 5,6, E. Kwan 7, H. Lenske 4, M. Marta 8, R. Raut 9, K. D. Schilling 1, A. Tonchev 7, W. Tornow 5,1, A. Wagner 1 1 Institut für Strahlenphysik, Helmholtz-Zentrum Dresden-Rossendorf 1314 Dresden 2 Technische Universität Dresden, 162 Dresden 3 Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 4 Institut für Theoretische Physik, Universität Gießen, Gießen 5 Triangle Universities Nuclear Laboratory, Durham, NC 2778, USA 6 Department of Physics, NC State University, Raleigh, NC 27695, USA 7 Physics Division, Lawrence Livermore National Laboratory, Livermore, CA 9455, USA 8 GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt 9 UGC-DAE Consortium for Scientific Research, Kolkata-798, India 1 Department of Physics, Duke University, Durham, NC 2778, USA Photon-scattering experiments at γelbe and at HIγS Data analysis Results Comparison of experimental results with model predictions

2 Electric dipole strength in nuclei Mo (γ,n) Absorption cross section σ γ 1 7 GDR Dipole strength function f 1 = σ γ /[3(π hc) 2 E γ ] f 1 (MeV 3 ) 1 8 S n E γ

3 Electric dipole strength in nuclei Mo (γ,n) Absorption cross section σ γ 1 7 GDR Dipole strength function f 1 = σ γ /[3(π hc) 2 E γ ] f 1 (MeV 3 ) 1 8 SLO Standard Lorentz SLO S n E γ

4 Electric dipole strength in nuclei Mo (γ,n) Absorption cross section σ γ 1 7 GDR Dipole strength function f 1 = σ γ /[3(π hc) 2 E γ ] f 1 (MeV 3 ) 1 8 SLO GLO Standard Lorentz SLO Generalized Lorentz GLO S n E γ

5 Electric dipole strength in nuclei Mo (γ,n) Absorption cross section σ γ 1 7 GDR Dipole strength function f 1 = σ γ /[3(π hc) 2 E γ ] f 1 (MeV 3 ) 1 8 SLO GLO TLO Standard Lorentz SLO Generalized Lorentz GLO Three-fold Lorentz TLO S n E γ

6 Electric dipole strength in nuclei Mo (γ,n) Absorption cross section σ γ 1 7 (γ,γ ) ELBE GDR Dipole strength function f 1 = σ γ /[3(π hc) 2 E γ ] f 1 (MeV 3 ) 1 8 SLO GLO TLO PDR? Standard Lorentz SLO Generalized Lorentz GLO Three-fold Lorentz TLO 1 9 S n E γ Implications for reaction rates in astrophysics and nuclear technology?

7 Photon scattering - feeding and branching Measured intensity of a γ transition: I γ (E γ, Θ) = I s ( ) Φ γ ( )ǫ(e γ ) N at W (Θ) Ω E e E, x Γ Integrated scattering cross section: I s = σ γγ de = 2J ( ) 2 x + 1 π hc Γ 2J + 1 Γ Γ Γ f branching Absorption cross section: Γ Γ 1 E1 strength: σ γ = σ γγ ( Γ Γ ) 1 B(E1) Γ /E 3 γ

8 Photon scattering - feeding and branching Measured intensity of a γ transition: I γ (E γ, Θ) > I s ( ) Φ γ ( ) ǫ(e γ ) N at W (Θ) Ω E e feeding E, x Γ Integrated scattering cross section: I s = σ γγ de = 2J ( ) 2 x + 1 π hc Γ 2J + 1 Γ Γ Γ f branching Absorption cross section: Γ Γ 1 E1 strength: σ γ = σ γγ ( Γ Γ ) 1 B(E1) Γ /E 3 γ

9 The bremsstrahlung facility at the electron accelerator ELBE dipole quadrupole Accelerator parameters: Maximum electron energy: 18 MeV Maximum average current:.8 ma Micro-pulse rate: 13 MHz dipole quadrupoles Be window steerer electronbeam dump radiator dipole quartz window beam hardener collimator polarisation monitor Pb accelerator hall Pb HPGe detector Micro-pulse length: 5 ps target experimental cave photonbeamdump PE Pb 1 m R.S. et al., NIM A 555, 211 (25) door

10 Detector setup at γelbe

11 3 Highest intensity (γ-rays/s/kev) γ-ray beam in the world Produces γ-rays by Compton backscattering inside the optical cavity of a storage-ring FEL Produces both linearly and circularly polarized beams Electron Accelerator Facility 18 MeV Linac pre-injector 18 MeV 1.2 GeV Booster Injector 25 MeV 1.2 GeV Storage Ring FELs: planar (linear pol.) and helical (circular pol.) For more details see: γ-ray beam parameters Values Energy 1 1 MeV Linear & circular polarization > 95% Intensity with 5% E γ /E γ > 1 7 γ/s Courtesy of C. Howell (TUNL)

12 Detector setup at HIγS Two HPGe detectors of 1% relative efficiency taken from the γelbe setup.

13 High-pressure gas target Technology as described in NIM A 68, 152 (29). Sphere of stainless steel; inner diameter 2 mm, wall thickness.5 mm. 1.7 g of 86 Kr at a pressure of about 6 bar.

14 Measurement at γelbe B E e kin = 11.2 MeV B Number of counts /7314 θ = 127 o B Kr + steel 7958 Identification of transitions in 86 Kr by comparison of spectra measured with the filled and with the empty steel container B 968/ steel 1116 R.S. et al., PRC 87, 2436 (213) E γ (kev)

15 Measurement at HIγS Counts Kr + steel horizontal detector M1 vertical detector E E beam = 6.5 MeV E γ (kev) Measurements at beam energies of 4.7, 5.1, 5.6, 6.1, 6.5, 7., 7.5, 8., 8.4, 8.7, 9. and 9.3 MeV. Identification of E1 and M1 transitions by comparison of spectra measured with the horizontal and with the vertical detector. All intense transitions in 86 Kr have E1 character. R.S. et al., PRC 87, 2436 (213)

16 Level densities in Mo isotopes Mo BSFG Experiment at E kin e = 3.8 MeV (MeV -1 ) EXP CT Experiment at E kin e = 13.2 MeV Back-shifted Fermi gas (BSFG) model Mo BSFG Constant-temperature (CT) approximation (MeV -1 ) CT 1 1 EXP G. Rusev et al., PRC 77, (28)

17 Unresolved strength in the quasicontinuum Y(γ,γ ) E e kin = 13.2 MeV La(γ,γ ) E e kin = 11.5 MeV Pb(γ,γ ) E e kin = 17 MeV Counts / 1 kev experimental spectrum Counts / 1 kev atomic background experimental spectrum Counts / 1.6 kev experimental spectrum 1 1 atomic background atomic background E γ E γ E γ Experimental spectra (corrected for room background, detector response and efficiency) and simulated spectra of atomic background. 89 Y data: N. Benouaret et al., PRC 79, 1433 (29). 139 La data: A. Makinaga et al., PRC 82, (21). 28 Pb data: R.S. et al., PRC 81, (21).

18 Scattering cross section including quasicontinuum Treatment of experimental spectra in energy bins = 1 kev. Correction of experimental spectrum for detector response: Intensities of full-energy peaks in the simulated spectra are adjusted to the experimental ones at the corresponding energy. Compton background and escape peaks are subtracted. This procedure is applied step-by-step for each energy bin, starting from S n and moving toward the low-energy end of the spectrum (spectrum-strip method). Calculation of the scattering cross section σ γ in a bin as σ γγ = I γ /(ǫ γ Φ γ N at ).

19 Unresolved strength in the quasicontinuum Y =.2 MeV peaks + cont La =.1 MeV 6 28 Pb =.1 MeV σ γγ (mb) 5 peaks σ γγ (mb) 5 peaks peaks + cont. σ (mb) 4 2 peaks Scattering cross sections averaged over energy bins, not corrected for branching, derived from the difference of the experimental spectrum and the atomic background (triangles) and from the resolved peaks only (circles). 89 Y data: N. Benouaret et al., PRC 79, 1433 (29). 139 La data: A. Makinaga et al., PRC 82, (21). 28 Pb data: R.S. et al., PRC 81, (21).

20 Simulations of γ-ray cascades Monte Carlo simulations of γ-ray cascades from groups of levels in 1 kev bins (G. Rusev, dissertation) Γ Γ 1 Γ 2 Γ E J π Level scheme of J = J ± 1, 2 states constructed by using: Backshifted Fermi-Gas Model with level-density parameters from T. v. Egidy, D. Bucurescu, PRC 8, 5431 (29) Wigner level-spacing distributions Partial decay widths calculated by using: Photon strength functions approximated by Lorentz curves (www-nds.iaea.org/ripl-2). E1: parameters from fit to (γ,n) data M1: global parametrisation of spin-flip resonances E2: global parametrisation of isoscalar resonances Porter-Thomas distributions of decay widths. Feeding intensities subtracted and intensities of g.s. transitions corrected with calculated branching ratios Γ /Γ.

21 Simulations of γ-ray cascades Kr Ground state La Ground state I γ (arb. units).5 Branching transitions transitions I γ (arb. units).5 transitions Branching transitions E γ E γ Simulated intensity distribution of transitions depopulating levels in a 1 kev bin around 9 MeV. Subtraction of intensities of branching transitions. 86 Kr data: R.S. et al., PRC 87, 2436 (213). 139 La data: A. Makinaga et al., PRC 82, (21).

22 Simulations of γ-ray cascades Kr La b (%) 6 4 b (%) Distribution of branching ratios b = Γ /Γ versus the excitation energy as obtained from the simulations of γ-ray cascades. Estimate of Γ and σ γ. 86 Kr data: R.S. et al., PRC 87, 2436 (213). 139 La data: A. Makinaga et al., PRC 82, (21).

23 Correction of spectra for branching Intensities of the elastic transitions in the simulated spectra are adjusted to the experimental ones at the corresponding energy. Intensities of the inelastic transitions are subtracted. This procedure is applied step-by-step for each energy bin, starting from S n and moving toward the low-energy end of the spectrum (spectrum-strip method). Calculation of the branching ratio b in a bin as b = i Γ i 2/Γ i i Γ. i Calculation of the absorption cross section σ γ in a bin as σ γ = σ γγ/b. Absorption cross sections of each bin are obtained by averaging over the values of all nuclear realizations (usually 1). Uncertainties are taken as a 1σ deviation from the mean.

24 Branching ratios from measurements at HIγS Response-corrected spectra. Simulated spectra of γ rays scattered by atomic processes. Subtracted spectra contain bunches of transitions to the ground state and to excited states. R. Massarczyk et al., PRC 86, (212).

25 Test of simulated branching ratios - measurements at HIγS b (%) Ba Red circles: Branching ratios deduced from measurements with monoenergetic γ rays at HIγS ( E 2 kev). Black squares: Distribution of branching ratios b = Γ /Γ versus the excitation energy as obtained from simulations of γ-ray cascades (values of 1 realizations). R. Massarczyk et al., PRC 86, (212).

26 Absorption cross section in 86 Kr Kr (γ, γ) data from ELBE σ γ (mb) S n (γ,γ ) peaks R.S. et al., PRC 87, 2436 (213)

27 Absorption cross section in 86 Kr Kr (γ,n) (γ, γ) data from ELBE (γ, n) data from HIγS JP:CS 337, 1248 (212) σ γ (mb) S n (γ,γ ) peaks R.S. et al., PRC 87, 2436 (213)

28 Absorption cross section in 86 Kr Kr (γ,n) (γ, γ) data from ELBE (γ, n) data from HIγS JP:CS 337, 1248 (212) σ γ (mb) S n (γ,γ ) (γ,γ ) peaks R.S. et al., PRC 87, 2436 (213)

29 Absorption cross section in 86 Kr 25 (γ, γ) data from ELBE 2 86 Kr (γ,n) (γ, n) data from HIγS JP:CS 337, 1248 (212) σ γ (mb) 15 1 (γ,γ ) corr 5 S n (γ,γ ) (γ,γ ) peaks R.S. et al., PRC 87, 2436 (213)

30 Absorption cross section in 86 Kr 25 (γ, γ) data from ELBE 2 86 Kr (γ,n) (γ, n) data from HIγS JP:CS 337, 1248 (212) σ γ (mb) 15 1 (γ,γ ) corr TLO TLO PLB 67, 2 (28) 5 S n (γ,γ ) (γ,γ ) peaks R.S. et al., PRC 87, 2436 (213)

31 Absorption cross section in 86 Kr 1 86 Kr (γ, γ) data from ELBE + (γ, n) data from HIγS σ γ (mb) 1 TLO 1 S n R.S. et al., PRC 87, 2436 (213)

32 Absorption cross section in 86 Kr 1 86 Kr (γ, γ) data from ELBE + (γ, n) data from HIγS σ γ (mb) 1 TLO 1 QRPA S n Calculations by N. Tsoneva R.S. et al., PRC 87, 2436 (213)

33 Absorption cross section in 86 Kr 1 86 Kr (γ, γ) data from ELBE + (γ, n) data from HIγS σ γ (mb) 1 2ph QPM TLO 1 QRPA S n Calculations by N. Tsoneva R.S. et al., PRC 87, 2436 (213)

34 Absorption cross section in 86 Kr 1 86 Kr (γ, γ) data from ELBE + (γ, n) data from HIγS σ γ (mb) 1 1 QRPA 3ph QPM 2ph QPM TLO S n Calculations by N. Tsoneva R.S. et al., PRC 87, 2436 (213)

35 Absorption cross sections in the N = 5 isotones 88 Sr, 9 Zr and 92 Mo σ γ (mb) Sr 9 Zr 92 Mo 88 Sr PRC 76, (27). 9 Zr PRC 78, (28). 92 Mo PRC 79, 6132 (29) R.S. et al., PRC 87, 2436 (213)

36 Summed dipole strength in stable even-mass N = 5 isotones σ i E i (MeV mb) Kr 88 Sr 9 Zr EXP 6 MeV < E i < 1 MeV 92 Mo 88 Sr PRC 76, (27). 9 Zr PRC 78, (28). 92 Mo PRC 79, 6132 (29). Σ 1 N = Z R.S. et al., PRC 87, 2436 (213)

37 Summed dipole strength in stable even-mass N = 5 isotones σ i E i (MeV mb) Kr QRPA EXP 88 Sr 9 Zr 6 MeV < E i < 1 MeV 92 Mo 88 Sr PRC 76, (27). 9 Zr PRC 78, (28). 92 Mo PRC 79, 6132 (29). Σ 1 N = Z R.S. et al., PRC 87, 2436 (213)

38 Summed dipole strength in stable even-mass N = 5 isotones σ i E i (MeV mb) Kr QRPA EXP 2ph 88 Sr 9 Zr 6 MeV < E i < 1 MeV 92 Mo 88 Sr PRC 76, (27). 9 Zr PRC 78, (28). 92 Mo PRC 79, 6132 (29). Σ 1 N = Z R.S. et al., PRC 87, 2436 (213)

39 Summed dipole strength in stable even-mass N = 5 isotones σ i E i (MeV mb) Kr QRPA EXP 2ph 3ph 88 Sr 9 Zr 6 MeV < E i < 1 MeV 92 Mo 88 Sr PRC 76, (27). 9 Zr PRC 78, (28). 92 Mo PRC 79, 6132 (29). Σ 1 N = Z R.S. et al., PRC 87, 2436 (213)

40 Transition densities in N = 5 isotones.2 86 Kr = 9 1 MeV Zr = 9 1 MeV r 2 ρ(r) (fm 1 ) < 9 MeV = 1 11 MeV = MeV.5 88 Sr = 9 1 MeV.5 r 2 ρ(r) (fm 1 ) < 9 MeV = 1 11 MeV = MeV.5 92 Mo = 9 1 MeV.5.1 < 9 MeV = 1 11 MeV.5 = MeV.1 < 9 MeV = 1 11 MeV.5 = MeV r (fm) r (fm) 5 Solid (dashed) lines: Neutrons (protons). Calculations by N. Tsoneva. R.S. et al., PRC 87, 2436 (213).

41 Summary First photon-scattering experiments on 86 Kr with bremsstrahlung at γelbe and with monoenergetic γ rays at HIγS. Determination of the dipole strength function at high excitation energy and high level density. Inclusion of strength in the quasicontinuum. Correction of the observed strength for branching and feeding by using statistical methods. Assignment of multipolarity E1 to the intense resolved dipole transitions. Enhanced strength observed in the energy range from about 6 to 1 MeV PDR. Comparison of the experimental data with results of QRPA and QPM calculations. The summed strengths between 6 and 1 MeV in N = 5 isotones seem to peak at proton subshell closures. No clear tendency with changing N/Z.